The present invention relates to a method for manufacturing semiconductor absolute pressure sensor units by employing anodic bonding process. The semiconductor absolute pressure sensor units thus manufactured are applied to such as in an automobile engine control field and a biomedical field.
Japanese patent application Laid-Open No. 61-202473 laid open on Sept. 8, 1986 discloses a method for manufacturing semiconductor absolute pressure sensor units in which a silicon sensor wafer and a silicon cap wafer are anodically bonded both in wafer state with borosilicate glass layers disposed therebetween by an application of a DC voltage between the silicon cap wafer and the borosilicate glass layers disposed around the circumference of the silicon sensor wafer, and thereafter the bonded silicon sensor wafer and silicon cap wafer is cut out to obtain the semiconductor absolute pressure sensor units.
However the bonding glass layers of the semiconductor absolute pressure sensor unit manufactured according to the above method include a substantial amount of sodium ions (Na.sup.+), in particular at the interface between silicon sensor chip and the borosilicate glass layer. The sodium ions include in the glass layers cause a drift to the sensor signal and chemical reactions in the glass layer to reduce the bonding strength and to induce a permanent stress to the pressure sensitive diaphragm of respective units, all of which inclusively reduce the accuracy and reliability of the resultant absolute pressure sensor.
Moreover a bonding strength of the glass layer bonding the silicon sensor chip and the silicon cap chip of the resultant semiconductor absolute pressure sensor units manufactured according to the above method and obtained from the center portion of the wafers is found out to be insufficient which is considered due to insufficient ion migration in the borosilicate glass layers at the center portion of the wafer during anodic bonding process because of the remoteness of the negative terminals which are connected at the circumference of the silicon sensor wafer.